1. Field of the Invention
The present invention relates to a process for the manufacture of a thermostructural composite material, involving the steps of making a fibrous preform from refractory fibers, and densifying the preform with a refractory matrix while providing a carbon interphase between the fibers and the matrix.
A thermostructural composite material consists of a fibrous preform and a matrix which, together provide high mechanical characteristics that can be maintained at elevated temperatures.
The process according to the invention can be applied to any type of fibrous preform. The latter can be made by packing short fibers, e.g. to provide a felt, or by winding fibers. The texture can also comprise a superposition or winding of one-dimensional (1D) plies or two-dimensional (2D) plies made from strands, cables or threads, each consisting of an assembly of fibers. It is also possible to use three-dimensional (3D) preforms, such as those obtained by needling superimposed or wound plies, or by three-dimensional weaving.
Refractory fibers forming the preform are chosen among those fibers that can withstand a temperature of at least 800.degree. C. in an atmosphere that is non-reactive for the fiber, without modification or fundamental deterioration. Examples of such fibers include carbon fibers and ceramic fibers (silicon carbide, alumina, zirconia or the like).
As to the refractory material forming the matrix, any refractory ceramic composition of the oxide, carbide, boride, nitride type or the like, as well as carbon, may be used.
The invention is more particularly aimed at composite materials having a ceramic or part-ceramic matrix, known as CMCs. These materials are employed in applications that require high-mechanical properties, such as in the manufacture of engine or jet components, or structural parts of space vehicles exposed to substantial heating effects.
In a CMC, the matrix in itself has a low breaking strain and tensile strength, a high susceptiblity to kirving, owing to its ceramic nature. In order to obtain a material that is resistant to shocks and crack propagation, the fiber-to-matrix link must be made weak, so that any crack arriving at the fiber-matrix interphase cannot continue across the fiber and cause the latter to break.
On the other hand, it is known that if high mechanical properties are to be obtained, and in particular a high resistance to flexing, then the bond between the fibers and the matrix must be rather high, in order to ensure that forces are transmitted to the fibers of the reinforcement.
2. Prior Art
A good compromise between these seemingly conflicting requirements has been found by interposing an intermediate coating, or interphase, between the fibers and the matrix. Such an interphase has a small thickness and a shear resistance which is lower than that of the matrix. Accordingly, when a crack in the matrix reaches the region of the fiber, the strains at the bottom of the crack will be released by the interphase. This interphase may e.g. be formed from a material having a laminar structure, such as laminar pyrolytic carbon or boron nitride, as described in U.S. Pat. No. 4,752,503.
According to the method taught in that document, the interphase is deposited on the fibers by chemical vapor deposition within the preform, before depositing in the matrix. This makes it necessary to carry out at least one operation between the steps of making the preform and forming the matrix. It will be noted that such operations involving chemical vapor deposition are generally long and require complex installations.